Networks that primarily utilize data link layer devices are often referred to as layer two (L2) networks. A data link layer device is a device that operates within the second layer of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer. One example of a common L2 networks is an Ethernet network in which end point devices (e.g., servers, printers, computers) are connected by one or more Ethernet switches or other L2 network devices. The Ethernet switches forward Ethernet frames, also referred to as L2 communications or L2 packets to devices within the network. As the Ethernet switches forward the Ethernet frames the Ethernet switches learn L2 state information for the L2 network, including media access control (MAC) addressing information for the devices within the network and the physical ports through which the devices are reachable. The Ethernet switches typically store the MAC addressing information in MAC tables associated with each of their physical interfaces. When forwarding an individual Ethernet frame, an ingress port of an Ethernet switch typically broadcasts the Ethernet frame to all of the other physical ports of the switch unless the Ethernet switch has learned the specific physical port through which the destination MAC address devices is reachable. In this case, the Ethernet switch forwards a single copy of the Ethernet frame out the associated physical port.
Recently, network service providers have offered systems that provide L2 connectivity between multiple, geographically separate L2 customer networks. That is, the L2 customer networks may be interconnected by the service provider to provide L2 connectivity as if the L2 customer networks were directly connected. One mechanism by which network service providers provide L2 connectivity to their customers is by utilization of Virtual Local Area Networks (VLANs). VLANs are a generic grouping mechanism for Ethernet packets that allow logical isolation of multiple L2 networks that share the same physical Ethernet ports.
To troubleshoot this L2 connectivity, a process referred to as Operations, Administration and Maintenance (OAM) generally provides the activities, tools, standards and other techniques that involve operating, administering and maintaining connectivity in the L2 computer network. One such OAM technique, referred to as Connectivity Fault Management (CFM), which is described in the Institute of Electrical and Electronics Engineers (IEEE) 802.1ag standard, entitled “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks—Amendment 5: Connectivity Fault Management,” which is hereby incorporated by reference as if fully set forth in this disclosure, may include a number of proactive and diagnostic fault localization procedures. For example, a network device operating in accordance with CFM may proactively transmit connectivity check (CC) messages at a predetermined rate to other switches within a maintenance association. That is, CFM allows an administrator to define a maintenance association as a logical grouping of devices within a L2 network that are configured to monitor and verify the integrity of a single service instance. The device may be located within a single region of the network or may be geographically separate from each other. A service instance may, for example, represent a portion of a provider network that a given customer can access to query a status of services delivered for that customer, such as the VLAN service discussed above. The CC messages include an identification of the maintenance association and are sent by each of the devices to provide connectivity verification to the other network devices within that particular maintenance association. The network devices in the maintenance association create and maintain a connectivity database specifying the group network devices from which periodic CC messages are expected to be received. After establishing connectivity with the other network devices in the maintenance association, monitor receipt of CC messages from each of the devices. If a CC message is not received from one of the network devices identified in the connectivity database within a configured time, the other network devices in the group may identify a failure of the device from which the CC message was expected to be received. This failure is commonly referred to as a “connectivity failure.”
In response to detecting a connectivity failure, the network devices of the maintenance association usually perform a number of reactive procedures to locate and then respond to the fault. For example, the network devices of the maintenance association may, in response to detecting the fault, issue loopback and link trace messages to locate the fault. Based on the location of the fault, the switches of the maintenance association may re-calculate paths based on the changed network topology or otherwise reconfigure the connectivity between the various network devices of the maintenance association so as to avoid the fault and restore connectivity between the network devices of the maintenance association. In some instances, restoring connectivity involves interface issues that require the network devices to bring down or otherwise disable the interfaces. As the rerouting of connectivity typically disrupts forwarding or delivery of services undergoing CFM monitoring, detection of a connectivity failure should only occur in response to faults that are unavoidable instead of false positive faults that could be avoided if properly managed by an administrator or other network operator through proper planning.